Giorgio Contento

Bifurcations in ship rolling : experimental results and parameter identification technique

The steady rolling response in a regular beam sea of a 1 : 50 scale model of a destroyer in the bare hull condition has been studied. The restoring moment of the hull features strongly softening characteristics. This has led to the experimental evidence of bifurcations with jump in amplitude and phase during the tests at two different wave frequencies. The high quality measurements allowed the analysis of the gathered data through a single-degree-of-freedom roll motion equation. Exact numerical solutions have been used to obtain reliable values of the coefficients of the mathematical model to be used for the roll motion simulation. Approximate perturbative solutions have then been used to obtain a description of the domains of attraction of the system. An analysis of the stability of the steady state solutions in the range of frequencies where bifurcations and jumps were detected coupled with the analysis of the change of shape of the domains of attraction with the frequency allowed the differences found between the two physical situations to be explained.

On the effectiveness of constant coefficients roll motion equation

Abstract As known, the rolling motion characteristics, amplitudes and accelerations, greatly influence the ability of a ship to operate and survive in bad weather. On the other hand, traditional computer codes for seakeeping calculations fail the forecasting of large amplitude rolling. There is a great need of using semi-empirical damping models and coefficients. This stresses the importance of campaigns of measurements as described in the paper, to get a deeper insight into the physical-mathematical modelling of the different contributions to rolling equation. Experimental tests on nonlinear rolling in a regular beam sea of a Ro-Ro ship model have been conducted by varying both the wave steepness and the wave frequency. The use of a parameter estimation technique, based on the least squares fitting of the stationary numerical solution of the nonlinear rolling motion differential equation, allowed to obtain informations on the damping model and on the linear and nonlinear damping coefficients. These exhibit a quite strong dependence on frequency that reduces the efficiency of constant coefficients rolling equation to simulate large amplitude nonlinear rolling. The results indicate that a good quality prediction model of nonlinear rolling cannot be based on constant coefficients time domain simulations. These can infact lead to incorrect estimates of rolling amplitudes even when the parameters have been obtained through high level parameter estimation procedures based on experimental data. The analysis indicates also a marked dependence of the effective wave slope coefficient on wave amplitude. The introduction of both these dependences on the rolling equation allows to reproduce the experimental results with great accuracy even at large amplitudes.

Nonlinear effects in 2D transient nonbreaking waves in a closed flume

Abstract The nonlinear effects deriving from the focussing of unidirectional waves propagating in a wave flume are analysed by direct fully nonlinear numerical simulation. The main aim of this study is to evidence the features of the wave–wave interaction in the spatial region, where the maximum elevation of the component waves of a given wave group is almost in phase. It will be shown that in the frame of the inviscid fluid hypothesis, the nonlinear interaction of waves leads to an evident space variation of the amplitude spectrum with a large magnification factor of the focused wave elevation compared to the result from linear superposition, a phase shift of the component waves occurs and part of the energy content is shifted to frequencies well above the highest frequency in the input spectrum. These features have basic consequences on the safety and design aspects of ships and marine structures: for instance, this regards the ringing response of large offshore structures. At the same time the availability of kinematic data from the simulation of large waves can be useful for the evaluation of the forces exerted on slender surface piercing bodies in inertia-drag regime method without semi-empirical stretching methods. Moreover the controlled generation of deterministic wave groups with a given frequency content, aimed at the development of new techniques for seakeeping tests, is one of the subjects of interest in ITTC (23rd International Towing Tank Conference).

Non-linear free surface induced pressure on a submerged horizontal circular cylinder at low Keulegan-Carpenters numbers

The interaction of regular waves with a horizontal circular cylinder at low KC Keulegan-Carpenter numbers is studied on an experimental basis. The main aim of the present study is to evidence the effects of the submergence of the cylinder on the higher-order frequency components of the pressure at the cylinder surface. It is shown that the bulk of the second and third harmonic term in the pressure is found on the lee side of the cylinder with peak values up to 25 and 15% respectively of the undisturbed pressure at the axis for the shallower cylinder condition tested. In the same case the higher-order terms of both the pressure and the free surface elevation exhibit saturation with increasing KC.

Experimental and Numerical Prediction of the Hydrodynamic Performances of a 65 ft Planing Hull in Calm Water

An extensive campaign of model and full scale experimental tests as well as of numerical computations, aimed at the prediction of the hydrodynamic performances of a 65 ft motoryacht in calm water and in waves, is undertaken within the framework of the Project SOPHYA Seakeeping Of Planing Hull YAchts, co-financed by Friuli-Venezia Giulia Region in the field of joint industrial and academic research. In this paper, selected results of the numerical computations conducted by HyMOLabUniversity of Trieste and by the University of Zagreb for the calm water case are presented. The RANS simulations carried out in combination with a semi-automatic optimized mesh generation tool, developed in this project within the OpenFOAM/foamExtend framework, are described. Different free surface capturing methods are employed and compared, with focus on numerical issues. The numerical results are compared with new experimental data obtained at the towing tank of the University of Naples within the project, with uncertainty assessment.